This invention relates generally to the control of the magnetization direction in multi-layer ferromagnetic devices by the use of a bias voltage. More particularly, this invention relates to the control of the direction of magnetization in multi-layer devices having a ferromagnetic/spacer/insulator/ferromagnetic configuration.
It is often desirable to control the direction of magnetization in multi-layer devices. A common type of such a device comprises two ferromagnetic electrodes that are separated by a thin layer of insulation. The standard practice for altering the magnetization direction is to apply an external magnetic field to the device. Typically this is accomplished by running a current through a conducting wire. The two ferromagnetic electrodes are configured to have different responses to magnetic fields such that the relative orientations are altered when in the presence of an external magnetic field.
Although such systems are reasonably useful, they have several significant limitations. For example, it is extremely difficult, if not impossible, to localize an external magnetic field to a high degree. By not being able to localize the magnetic field, the overall efficiency and usefulness of the magnetic devices is hindered when used in applications such as magnetic sensors, digital read heads and nonvolatile memory devices. In particular, novel magneto-electronic devices such as GMR (giant magnetoresistance) and TMR (tunneling magnetoresistance) systems which consist of F1/S/F2 and F1/I/F2 structures (F1, F2: ferromagnets, S: spacer, I: insulator layer) are actively being studied by many researchers for the applications of magnetic sensors, digital read heads, and nonvolatile memory. Most of these structures are fabricated to respond to external magnetic fields. If the magnetization direction could be controlled by an applied voltage which can be easily localized, numerous new applications and devices could be developed.
There have been attempts to create different types of ferromagnetic devices with improved qualities but these have their drawbacks. For example, U.S. Pat. No. 5,764,567 issued to Parkin, discloses a four-layer structure wherein the two outer layers are comprised of ferromagnetic materials. Between the two outer layers are an insulating layer and a spacer layer. This four-layer structure forms a magnetic tunnel junction, creating a quantum-mechanical tunneling of the charge that is present between the ferromagnetic electrodes. One of the ferromagnetic layers is fixed in a preferred position, while the other ferromagnetic layer is free to rotate its magnetization direction. When an external magnetic field is applied to the structure, a magnetoresistance response if formed. Unfortunately, however, this device still relies upon the presence of an external magnetic field to operate successfully. Although the '567 reference describes the use of an additional current that is applied to the system, this voltage is intended to be extremely small (applying a voltage of &lt;0.1 volts) and is used primarily to aid in the study of the resulting current.